Somewhere around two in the morning, while the rest of the forest is silent, a birch tree lets go. Its branches sag. Its leaves tip downward. The whole crown settles a few centimeters closer to the ground, and it stays that way until an hour or so before dawn, when the tree slowly lifts itself back up to meet the light.
Nobody had ever watched this happen until a team of scientists pointed a laser at a tree and left it running all night.
Quick facts
- In 2016 researchers laser-scanned two silver birch trees, one in Finland and one in Austria, from sunset to sunrise.
- Both trees drooped through the night by up to 10 centimeters, reaching their lowest point around or shortly before sunrise.
- The movement is driven by water pressure inside the tree’s cells, which falls when photosynthesis stops.
- A follow-up on 22 species found that not every tree sleeps the same way, and some twitch in slow two-to-four-hour cycles all night long.
An old idea, finally measured
People have known for a long time that some plants move at night. Bean seedlings fold their leaves at dusk. Clover closes. Charles Darwin spent years cataloguing these “sleep movements” in small plants and wrote a whole book about it in 1880. But those are little plants on a windowsill, and you can see them move.
A tree is a different problem. If a birch tree sagged a few centimeters overnight, how would anyone know? You cannot photograph it in the dark without light, and light is exactly the thing you are not supposed to add.
The answer turned out to be infrared. A terrestrial laser scanner sweeps an invisible beam across a scene and records millions of points, each one a precise distance measurement, building a three-dimensional map accurate to well under a centimeter. Plants do not react to infrared the way they react to visible light, so the scanner can watch a tree all night without waking it up.
Two birches, two countries, one long night
Eetu Puttonen of the Finnish Geospatial Research Institute, together with András Zlinszky of the Hungarian Academy of Sciences and Norbert Pfeifer of TU Wien, chose a silver birch in Finland and another in Austria. They picked nights near the equinox, when day and night are roughly equal, and waited for calm weather with no wind and no dew, so that nothing but the tree itself could move the leaves. Then they scanned, again and again, from sunset to sunrise.
The point clouds told the same story in both countries. Through the evening the branches and leaves gradually sank. The Finnish tree reached its lowest point around sunrise; the Austrian tree bottomed out about an hour and a half before. The total drop was as much as 10 centimeters on trees about five meters tall. That does not sound like much until you picture it: the entire crown of a tree, every branch and leaf, sinking together in the dark and then rising again by morning, like a slow breath taken once a day.
“The changes are not too large,” Puttonen said at the time, “but they were systematic and well within the accuracy of our instruments.”
What is actually happening inside the tree
Trees do not have muscles. What holds a leaf up and a twig firm is turgor, the water pressure inside every cell pressing outward against the cell wall, the same pressure that makes a fresh lettuce leaf crisp and a wilted one limp. During the day, photosynthesis keeps that pressure high. When the light goes, the process winds down and the pressure eases. Cell by cell, the whole tree relaxes, and the branches, no longer held quite so taut, settle under their own weight. At dawn the process reverses.
Whether the tree wakes because the sun touches it, or because an internal clock tells it the sun is about to, the researchers could not say. The Austrian birch started lifting before sunrise, which hints at a clock. Sunflowers, as it happens, do the same thing, turning to face the east while it is still dark. Trees may be keeping time too.
Not all trees sleep the same
The birch study raised an obvious question, so Zlinszky went back the following year with a bigger cast. In September 2016, at a nursery in Hungary, the team scanned 22 small trees of different species through one twelve-hour night, all at once. Every tree changed shape. But a clean, birch-style sag turned out to be rarer than expected. A honey locust moved the most, its crown shifting as much as 9 centimeters. Horse chestnut and maple showed a tidy daily rhythm of around 2 centimeters. A Himalayan birch and an olive tree barely moved at all, staying below the half-centimeter threshold the scanner could reliably detect.
Then came the strange part. Several trees were not only sinking and rising over the night. They were also pulsing, in slow cycles of two to four hours, with each cycle moving the crown by half a centimeter to a centimeter. The movement did not track temperature, humidity, light or wind. It seemed to be coming from inside the tree.
Zlinszky and his colleague Anders Barfod have since proposed that these pulses could be the signature of a pumping mechanism nobody has found yet: water moving up a tree in waves rather than a steady stream, each surge lifting the branches a fraction and each lull letting them down. If they are right, a tree at night is not just resting. It is quietly, rhythmically working, in a way we have only just begun to measure.
A forest, breathing
Ten centimeters is nothing. It is the width of a hand. But multiply it by every tree in a forest and something shifts in how you picture the place. A wood at three in the morning is not a set of statues waiting for daylight.
It is a crowd of enormous living things, each one slack and low, each one drawing itself back up before the first light reaches it. The next time you walk out early enough to catch the trees still gray and quiet, you are looking at a forest that has just, in its own way, woken up.
Do trees really sleep at night?
Trees do not sleep the way animals do, but they have a measurable day-and-night rhythm. Laser scans have shown birch trees drooping by up to 10 centimeters overnight and lifting again by morning, driven by changes in water pressure inside their cells.
Why do tree branches droop at night?
When photosynthesis stops after dark, the water pressure that keeps cells firm eases slightly, and branches and leaves settle under their own weight. The pressure rebuilds around dawn and the crown rises again.
Do all trees move at night?
A 2017 study of 22 species found that all of them changed shape overnight, but by very different amounts. Honey locust moved up to 9 centimeters, maples and horse chestnuts about 2, and some species barely moved at all.
For more on the hidden lives of trees, we have written about Pando, the aspen that is secretly one tree, the oldest tree on Earth, and the strangest things plants do in spring. There is much more in our Nature section.
Sources: Puttonen et al., “Quantification of Overnight Movement of Birch (Betula pendula) Branches and Foliage with Short Interval Terrestrial Laser Scanning,” Frontiers in Plant Science, 2016. Zlinszky, Molnár and Barfod, “Not All Trees Sleep the Same,” Frontiers in Plant Science, 2017. Zlinszky and Barfod, “Short interval overnight laser scanning suggest sub-circadian periodicity of tree turgor,” Plant Signaling & Behavior, 2018.
